Polyacrylic acids and polymethacrylic acids have long been known and are commercially available as, inter alia, Sokalan.RTM. (BASF AG). These polymers are usually used as additives for reduced-phosphorus and phosphate-free detergents and cleaning agents or as dispersants for solids (for example for preventing coatings in water treatment).
Suitable polymer compositions which can be used for the production of hollow articles by extrusion blow molding require, inter alia, high molecular weights in the case of polyamides.
In blow molding, a tube of polymer melt is generally extruded and is suspended between the two half-shells of the opened mold. The mold is then closed and the polymer tube is pressed against the mold by means of internal gas pressure, cooled and removed from the mold.
In this processing, it is essential that the polymer tube does not tear during extrusion in the periods when it is freely suspended between the molds, so that the molding process can be completed. It is also desirable that the tube does not sag, since this results in smaller wall thicknesses in the upper half and larger wall thicknesses in the lower half. Hollow articles having different wall thicknesses are unsuitable for use since the strength is as a rule limited by the point having the smallest wall thickness.
Particularly problematic is the danger of breaking of the melt or sagging in the case of glass fiber-reinforced polymer melts, since these have a high density (and hence a high tensile force on the upper part of the polymer tube) and at the same time the maximum extensibility of the polymer melt required to break is smaller in the case of these reinforced materials.
Both factors are governed by the melt rigidity, which depends primarily on the melt viscosity. What would be ideal is a high melt viscosity with little shearing--ie. after extrusion--but a low melt viscosity under a steep shear gradient--ie. in the processing extruder.